Freshly cut lumber contains a substantial amount of moisture which must be removed before the lumber is suitable for commercial use as construction material, decorative material, or even as firewood. A number of factors affect the drying process, including the species of wood to be dried, the season of the year in which cutting takes place, the ambient temperature and humidity of the region in which drying is attempted, and the desired moisture content of the finished product, which varies with the species of wood and its planned application. In addition, the drying process must be carefully monitored and controlled to avoid causing problems such as warping, case hardening, shrinkage, checking, and cellulose destruction, any or all of which may occur in the lumber upon attempts to hasten the drying process under less than optimum conditions.
Many methods have heretofore been tried to effect the drying of lumber, ranging from open-air drying, a process which may take anywhere from two to four months up to a year or more depending on the species of wood, and useless when the relative humidity is above 60%; to kiln-drying where heated air is circulated around and through a stack of lumber, which process may take one to two months; to various methods in which drying is effected in a vacuum. U.S. Pat. No. 2,296,546 to Toney discloses a method in which steam is introduced into one chamber under vacuum conditions to remove some of the moisture content of the lumber, followed by kiln-drying the partially dehydrated lumber in a second chamber to reduce the moisture content of the lumber further. Thus, the stack of lumber must be transferred from one vessel to another during the drying process and the method is only able to achieve a final moisture content of 14-16% in the lumber, this figure being unacceptably high for many applications. In addition, the temperature of the steam introduced into the first chamber, 220.degree. F., is high enough to risk cellulose damage in the lumber. Cold water is circulated through pipes within the first chamber to promote circulation and to assist in reducing the pressure.
U.S. Pat. No. 2,830,382 to Petersen, discloses a method wherein lumber to be dried is stacked in a chamber without providing space between the boards. The equipment used includes vacuum means in conjunction with a refrigeration unit, said unit preferably being mounted on the outside of the drying chamber with pipe coils extending into the chamber for maintaining the temperature inside the chamber at or slightly below 35.degree. F. At this temperature and the low pressure provided by the vacuum, oils and other extractives tend to evaporate from the wood, leaving the water, which must then be removed by air-drying. The process must be halted at scheduled intervals to defrost the coils and remove accumulated ice and other congealed extractives. Warm air is provided for this defrosting stage to help defrost the coils and to prevent the lumber from freezing. Since air-drying is required, the moisture content becomes dependent on the length of air-drying time. A similar device is disclosed in U.S. Pat. No. 3,574,949 to Farnsworth, except that the refrigeration system is disposed inside the drying chamber to transform moisture extracted from the lumber into ice crystals and then by sublimation into water vapor, requiring a critical sublimation balance to be maintained. Heat is supplied to the drying chamber to help effect sublimation and to scavenge any air remaining in the chamber which is not removed by the vacuum pump.
Typically, existing vacuum drying devices have encountered difficulty in managing the large amount of moisture which is liberated during the drying process, and the processes must be halted in order to purge the systems of moisture, whereupon the cycles are repeated to effect further drying. If the drying process is not carefully monitored and controlled, the lumber may suffer from warping, case hardening or any number of the previously listed hazards, resulting in a lowering in the grade of the lumber, lost time and energy, and possible destruction of valuable materials. In addition, it has been found to be very difficult or even impossible to obtain a final moisture content below 5-6% in the lumber without damaging it, and where obtainable, as discussed in U.S. Pat. No. 3,921,309 to Nakayashiki, where the specification relates examples of dried lumber with moisture contents ranging from 0-10%, the hot air and vacuum steps must be repeated from 3 to 8 times and the entire sequence of steps must be repeated from 4 to 20 times, thus necessitating close monitoring of the process along with the increased costs of such monitoring.
The moisture content of green lumber is mainly water with small percentages of sap and other constituents present. The primary objective in the drying of lumber is to remove or reduce this water content in order to provide a suitable product. Thus, while it is well known that the boiling point of water is lowered by reducing the atmospheric pressure through vacuum means, and some prior art references in this area have used reduced pressure and elevated temperatures to boil the water out of the lumber, none of the processes have been able to accomplish complete drying in a single drying vessel without stopping the process to remove the accumulated vapor. In addition, prior devices have used temperatures high enough to cause cellulose destruction in the lumber, which may render the lumber useless for anything except firewood. The equipment which uses very low temperatures to transform the extracted moisture into ice must be halted during the drying process to remove the accumulated ice.